Disk files (or "drives") are information storage devices which utilize a rotatable disk with concentric data tracks containing the information, a head (or "transducer") for reading or writing data onto the various tracks, and an actuator connected to the head for moving it to the desired track and maintaining it over the track centerline during read or write operations The movement of the head to a desired track is referred to as track accessing or "seeking", while the maintaining of the head over the centerline of the desired track during read or write operations is referred to as track "following".
In disk files which have a relatively high density of data tracks on the disk, it is necessary to incorporate a servo control system to maintain the head precisely over the desired track during read or write operations. This is accomplished by utilizing prerecorded servo information either on a dedicated servo disk or on sectors angularly spaced and interspersed among the data on a data disk. During track following, the servo information sensed by the read/write head (or the dedicated servo head if a dedicated servo disk is used) is demodulated to generate a sampled position error signal (PES) which is an indication of the position error of the head away from the track centerline. The PES is used in the servo feedback loop to generate a control signal to the actuator to move the head back to the track centerline. A description of operation of a general disk file servo control system is given by R. K. Oswald in "Design of a Disk File Head-Positioning Servo", IBM Journal of Research and Development, November 1974, pp. 506-512.
There are several causes for the head being off the track centerline during track following and which thus contribute to the PES. Certain position error components are of a low frequency, that is of a frequency close to the frequency of rotation of the disk, and are repeatable with disk rotation. For example, if the disk is not precisely centered over the axis of rotation of the spindle motor, the circular tracks will have an eccentric shape relative to the axis of rotation. This will cause a repeatable disk "runout" error at the same frequency as the rotation of the disk. Similarly, even if the disk is precisely mounted about the spindle motor axis, but the spindle motor axis "wobbles" during rotation, this will also generate repeatable errors, typically at the fundamental and second harmonic frequencies of the disk rotational frequency. The problem of repeatable error is magnified in disk files which have multiple disks since each disk, and indeed each disk surface, will have its own unique repeatable error signature.
Several techniques have been used to remove this repeatable error from the PES so that the PES contains only nonrepeatable generally high frequency components. Such techniques typically use either the measured or predicted repeatable error to generate a signal which modifies the control signal to the actuator so that the head follows the true shape of the track centerline.
In one technique, as disclosed in co-pending patent application, Ser. No. 606,009, assigned to the same assignee as this application, the control signal to the actuator, which is a direct function of the acceleration of the head, is integrated twice to generate a pseudo head position signal. This signal is summed with the PES and the resultant signal is twice differentiated to generate a signal which is stored for at least one disk rotation. The stored signal is then later read out and combined with the actuator control signal.
U.S. Pat. 4,135,217 to Jacques et al. discloses a servo control system in a disk file of the type which utilizes an external transducer for generating signals representing a coarse position of the head. These coarse position signals are representative of the repeatable error and are stored in a memory device. During track following the repeatable error information is recalled from memory and used to generate an error signal to control the head and cause it to follow the true centerline of the desired track.
An article entitled "Track Locating and Following Apparatus For A Flexible Disk File" by R. C. Jahnke, IBM Technical Disclosure Bulletin, Vol. 23, No. 2 (July 1980) pp. 738-742 and an article entitled "Self Calibrating Disk Storage Apparatus" by D. E. Griffiths and H. E. VanWinkle, IBM Technical Disclosure Bulletin, Vol. 19, No. 6 (November 1976) pp. 1991-1992, both disclose methods of improving the track following characteristics of the head by sensing and storing information relating to the true profile of the tracks and later using that stored profile information as part of the control signal to the actuator.
An article entitled "Disk Runout Accommodation" by J. P. Mantey, IBM Technical Disclosure Bulletin, Vol. 21, No. 7 (December 1978) pp. 2688-2691, discloses a method for predicting the repeatable error by using the error signal and its rate of change to generate a disk runout estimator signal for input to the actuator control signal.
The prior techniques for removing repeatable error from the PES typically require at least four to five disk revolutions, in order to assure system stability, before the measured or estimated repeatable error signal can be applied to the control signal. More importantly, it may be necessary to store many track profiles for each disk or to utilize complex algorithms to estimate the profile of a track at a particular radius based upon the measured profile of a track at another radius. In the case of disk files with multiple disks this requires a substantial amount of memory storage space.